This invention relates to magnetrons and it relates particularly to a magnetron which has a rotatable tuning member, for example a spin tuned magnetron. A spin tuned magnetron generates a microwave output of varying frequency and is useful in radar applications requiring frequency agility.
A known spin tuned magnetron is shown in FIG. 1 of the accompanying drawings.
It comprises an anode 1 in the form of a number (typically, sixteen) of vanes 2 which surround, and project radially inwards, towards a cathode 3. Only two such vanes are illustrated in FIG. 1. A spinner 4 is mounted on bearings 5 for rotation about the longitudinal axis of the cathode and is provided with a number of slots 6 arranged around the axis of rotation. As the slots pass across successive anode cavities, defined between adjacent vanes, the resonant frequency of the anode circuit varies and one complete cycle back and forth across the available tuning range occurs each time a slot moves past a cavity. Thus, in the case of an anode consisting of sixteen evenly spaced vanes and a spinner having eight evenly spaced slots sixteen complete frequency cycles are obtained for each rotation of the spinner. It has been customary to generate an indication of the instantaneous magnetron frequency. This indication can be utilized to assist in tuning the receiver local oscillator to the transmitted frequency.
The indication can be derived from the spinner drive. However, because the drive rotor is coupled to the spinner magnetically, through the vacuum envelope of the magnetron (7, in FIG. 1), it may be subject to a phase lag with respect to the spinner, giving rise to error. An alternative approach involves using a capacitive transducer comprising two sets of meshing plates, one set being mounted on the spinner, inside the envelope, and the other set being fixed in relation to the spinner. The plates are divided into the same number of segments as there are anode cavities and so a variation of capacitance, resulting from rotation of the spinner relates directly to the variation of magnetron frequency. The transducer is relatively bulky and can be susceptible to interference from local fields within the magnetron.
Hall effect devices, responsive to changes of magnetic field caused by rotation of the spinner, have also been used, though again these devices may be susceptible to interference from local fields.
A preferred approach is described in our copending European patent application No. 86 300940.3. Rotation of the spinner is monitored by sensing movement of markings, formed circumferentially at its surface, past a window in the magnetron vacuum envelope. A processing circuit counts pulses generated by a detector as the markings pass the window and evaluates the instantaneous magnetron frequency.
In normal operation, the magnetron transmits pulses of radiation periodically, typically at a p.r.f. of about 5 kHz, whereas the frequency agility rate may be of the order 1200-1600 Hz. In general, transmissions occur at distinctly different phases in successive frequency cycles and so, in effect, pulses are transmitted in accordance with a psuedo-random sequence of frequencies.
In some operational circumstances, particularly if a receiver, associated with the magnetron, is remote from the transmitter, it is desirable to transmit pulses only at preselected frequencies, and it is one aim of the present invention to provide a magnetron whereby this objective may be satisfied.